U.S. Pat. No. 4,712,906 issued Dec. 15, 1987 to R. Borruso and J. Butler shows a color printer having an electrophotographic endless web entrained about a series of rollers. Electrostatic images representing color components of a desired multicolor image are formed by uniformly charging the web and exposing it using an LED printhead. Consecutive images (or pairs of images) are toned with different colored toners and then transferred in registration to a transfer surface brought into contact repeatedly with the images by a transfer drum.
Registration of the color images is achieved in part by an encoder which monitors the angular position of a roller associated with the printhead. The encoder triggers exposure of the web according to that angular position. The printhead roller is connected by gearing or a timing belt to the transfer drum to assure that the transfer surface is presented in registration with the beginning of the first image. The accuracy of placement of subsequent images on top of the first image is dependent upon consistency in the movement of the web between the printhead and the transfer station. With this structure, small errors associated with slippage of the web, errors in the drive train between the transfer drum and the printhead roller, and the like, cause small errors which accumulate over the course of formation of a set of multicolor images. While these errors are relatively small and not unacceptable for much color work, they prevent forming images of highest quality with ordinary web drive controls.
U.S. Pat. No. 4,025,186 shows an endless web electrophotographic member having a series of perforations along one edge which perforations are sensed to control actuation of work stations along the web. U.S. Pat. No. 4,477,176 shows a similar approach for controlling superimposed multiple image apparatus, for example, a multicolor image copier.